One form of method of measuring unbalance of a vehicle wheel when freely and rotatably mounted on a motor vehicle involves rotating the wheel at a measurement speed of rotation that will permit the unbalance measurement operation to proceed. A mark on the wheel is sensed for detecting the speed of rotation and/or the rotational angle of the wheel, and any force resulting from unbalance of the wheel is ascertained, relative to its rotational angle, during the unbalance measuring operation, by centrifugal force measurement. Such a method, and an apparatus for carrying it into effect, is disclosed in the journal `Autohaus` 9/1982, pages 912-920. The wheel being measured, in order to be freely rotatable, is suitably lifted by means of a jacking device which can be formed at the same time as a force measuring stand.
When dealing with the driving wheels of a motor vehicle, the wheel to be measured may be driven by means of the vehicle engine; while, when dealing with the non-driving wheels of a motor vehicle, the wheel can be driven by means of a friction wheel forming part of a movable piece of equipment and which can be brought into frictional engagement with the motor vehicle wheel to drive it at the speed required for the measuring operation. To carry out the measuring operation itself, the friction wheel is moved away from the motor vehicle wheel again and the unbalance measuring operation is performed as the wheel rotates freely and is in the course of running down.
To provide a selective measurement method, the wheel to be measured may be provided with a reflective reference mark and wheel rotation can then be detected by means of an opto-electronic system such as an infra-red system, for example as is to be found in DE 38 28 724 A1, corresponding to U.S. Pat. No. 4,956,998.
In that procedure, as the wheel rotates, the reference mark passes two adjacently focussed infra-red light beams, a receiver receiving pairs of light reflections from the reference mark. The sequence of double reflections and the measured periods of time between successive double reflections provides information about the direction of wheel rotation, about the mean wheel speed/angular speed during the last period of time of a revolution of a wheel, and changes in the mean speed between the last revolutions. On the basis of that information, and taking into account the variation in speed of rotation in the preceding revolutions of the wheel, the probable period of time can then be extrapolated for the next respective measurement period during a wheel revolution and that anticipated period of time can be so subdivided that the portions thereof correspond to assumed constant rotational angles of the wheel during the same period.
Measurement of centrifugal forces or variations in centrifugal force produced by the wheel which is supported on the jacking device, for example as indicated by a measuring stand during the divided portions of the period of time, and evaluation of the measurement result using procedures which are known in relation to static balancing machines, make it possible to provide for selective measurement of the unbalance vector involved.
When the detected direction of wheel rotation is taken into consideration, the correction or balancing position at which a correction or balancing weight is to be fixed can be indicated, independently of the direction of rotation required to arrive at the appropriate position.
However, inaccuracies can occur by virtue of the fact that the drop in the speed of rotation of the wheel when it is in the phase of rotating freely, and thus gradually slowing down in the actual measuring operation, is not linear. The decrease of rotational speed depends upon a number of factors such as friction, a rubbing brake, the effect of air such as air turbulence, and the like. When the wheel having its unbalance measured is a driving wheel and when the wheel is driven by the vehicle engine to bring it up to the speed of rotation required for the measuring operation, variations in the speed of rotation of the wheel can occur due to the engine not running in a totally smooth fashion or due, for example, to the influence of the differential unit or universal joints in a drive shaft. When such variations in speed of rotation are extrapolated to the period of time involved in the next following revolution of the wheel, they can result in defective evaluation of the measurement signals in terms of ascertaining the appropriate sizes of balancing weights and the correct angular positions for fitting those balancing weights.
It is also difficult for an operator to achieve good alignment of an optical sensing system with a reference mark on a wheel to be measured, and that can also give rise to further errors in the measurement values obtained.